Fix merge_features crash on empty DataFrame input

[Copilot]

merge_features raised ValueError: zero-size array to reduction operation maximum which has no identity when passed an empty DataFrame, because it unconditionally called values.max() before checking whether the array had any elements.

Changes

• deimos/alignment.py: Added an early-return guard in merge_features — if the input DataFrame is empty, return it immediately before any distance computation:

  if len(features.index) == 0:
      return features

• tests/test_alignment.py: Added test_merge_features_pass_none and test_merge_features_empty to cover the None and empty-DataFrame paths.

Example

import pandas as pd
import deimos

empty_df = pd.DataFrame(columns=['mz', 'intensity'])

# Previously raised ValueError; now returns the empty DataFrame
result = deimos.alignment.merge_features(empty_df, dims=['mz'], tol=[5e-6], relative=[True])
assert len(result) == 0

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Issue: merge_features fails with empty DataFrame</issue_title>
<issue_description>## Description

The deimos.alignment.merge_features() function raises a ValueError when called with an empty DataFrame, even when the DataFrame has the correct column structure.

Steps to Reproduce

import pandas as pd
from deimos.alignment import merge_features

# Create empty DataFrame with correct structure
empty_df = pd.DataFrame(columns=['mz', 'intensity'])

# This raises ValueError
result = merge_features(
    empty_df,
    dims=['mz'],
    tol=[5e-6],
    relative=[True]
)

Expected Behavior

The function should handle empty DataFrames gracefully and return an empty DataFrame with the same structure, or raise a more informative error message about the input being empty.

Actual Behavior

ValueError: zero-size array to reduction operation maximum which has no identity

The error occurs in deimos/alignment.py:411 when attempting to calculate max_tol = tol[i] * values.max() on an empty array.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Issue: merge_features fails with empty DataFrame #44

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